The present invention relates to a method of providing integrated optical systems made up of precision molded plastic pieces that are quickly assembled and bonded into a one piece system without use of external alignment aids and references. Assembly to the necessary tolerances is accomplished by virtue of keyed and slotted parts and reference surfaces molded directly into the individual pieces to the precision necessary to effect the alignment. The molds are designed such that all optical surfaces are referenced to the built in alignment aids and the resulting parts slip together as rods into a tube, maintaining axial and radial alignment by virtue of keyed and slotted guides. Longitudinal alignment is accomplished by contact of reference surfaces which preclude further insertion of one component into another. Once assembled, a single drop of bonding agent will lock two such pieces together permanently.
The individual molded parts each include two optical surfaces. In the current embodiment these surfaces comprise refractive or diffractive optics. Future embodiments may also include mirrored surfaces, but these embodiments will require extra steps in manufacturing to coat the mirrored surfaces. Refractive surfaces may include aspheres or anamorphic devices of a variety of shapes. Diffractive binary elements of up to eight layers and one micron precision have successfully been manufactured. Refinement of the processes may improve this performance. Simple systems comprised of up to four surfaces can be manufactured from two molded pieces. More complex optical systems may require three or more molded pieces, each pre-engineered to provide the precision alignment features and reference surfaces allowing simple assembly without additional mechanical or optical aids.
In laser diode collimating and aberration correction (LDCC) molded plastic optical systems, edge emitting laser diodes experience an inherent large divergence angle of the emitted light (called the fast divergence axis) and a less divergent angle in the orthogonal direction (called the slow divergence axis), such that the effective size of the illumination area grows very rapidly as the light propagates away from the emitter. For some applications, it would be desirable to have the energy emitted from the diodes to be collimated (minimize the divergence as a function of distance from the laser diode) in a beam that is as small and as uniform in shape (circular) as possible. This is the case in numerous applications, such as laser imaging/illumination, fiber optic communications, and in laser bar code scanners where the successful operation of the device is dependent upon whether there is sufficient brightness (laser light intensity confined to a small beam size) of light present at the desired plane at a location removed from the laser diode. The more divergent a laser beam is, the more rapidly the intensity decreases over a constant cross sectional area normal to the direction of propagation. Typically, the more divergent the laser beam, the more difficult it is to collimate the beam.
Highly divergent sources, including laser diodes, usually require optics that produce non-spherical wave fronts (aspheric refractive surfaces or complex phase function diffractive optics) to achieve diffraction limited collimated performance. The aspheric surfaces are usually defined by the radius of curvature (as if the surface was a perfect sphere) and the conic constant (the deviation of the surface from the perfect sphere). The large divergence of the laser diode fast axis usually requires a highly aspheric surface that is often very difficult to fabricate. As the surface approaches a sphere it is usually easier to fabricate but the degree of collimation decreases due to aberrations.
In addition, the fast and slow axes divergences are different which causes ellipticity in the cross sectional shape of the beam. This problem is linked to astigmatism. Astigmatism manifests itself as a difference in the origination point of the divergence of the fast axis relative to the slow axis. Because of the astigmatism, any optical lens system that attempts to collimate the beam must possess different optical power in the two orthogonal axes because the fast axis divergence is typically three to four times greater than the orthogonal slow axis divergence. In order to achieve collimation, an anamorphic lens (a single lens that has different optical power in two orthogonal axes) or a pair of cylindrical lenses (termed a cross cylindrical lens system) is required. To fabricate aspheric anamorphic lenses is difficult and costly. Therefore, aspheric crossed cylinders are often employed to collimate the beam. While either of these approaches is capable of collimating the laser output, special attention must still be given to the selection of the optics to achieve a circular cross sectional shape for the beam.
A further consideration with regards to the performance of the optical system is that the system should be achromatic over a specified waveband. The lenses in the system must be achromatized such that different wavelengths of laser diode light still focus to the same point. This is necessary to compensate for laser wavelength instabilities inherent in the laser diode due to electrical and thermal fluctuations. Added versatility can also be achieved by achromatizing the system because once this is done, a single system can often serve a wide waveband instead of having to fabricate many different systems for every different wavelength of laser diode.
Lastly, it is desirable to minimize the diameter of the collimated beam. The smallest beam diameter is usually a function of how closely the optical surfaces or lenses can be placed to each other. Since all conventional refractive lenses rely on material thickness to retard the wave front and thereby bend and focus the light, the conventional lenses require some finite spacing away from the laser diode which produces a certain collimated beam diameter. While it is usually easiest to manufacture and align optical systems that contain components that are on the order of millimeters in diameter, it is possible to decrease the element sizes through the use of diffractive optics and micro-diffractive optics. Diffractive optics are one way to reduce material thickness and permit more elements within less space. Micro-diffractives (sub millimeter sizes) are another method but these lenses are more difficult to align. The result is that the optical elements can be located closer to the laser diode and the result is a smaller diameter output beam.
The laser systems of known systems rely on individual elements that must be aligned one to another and then bonded into a complete system. This approach is both labor and time intensive which increases cost and prohibits the mass production of devices. The plastic molded approach only requires a single high fidelity alignment of the actual mold and once completed, the replicated parts maintain this precision. In addition, the optical design has been optimized and reduced in size to the point where the collimated beam is as small as possible without having to violate the integrity of the laser diode.
Most previous optical systems for a wide variety of applications have been constructed of discrete components that must be mechanically mounted and aligned by various optical and interferometric means. These optical surfaces of these individual components are generally manufactured by grinding and polishing of glasses, metals or plastics, and in some cases by molding of plastic. The assembly is typically very manpower intensive and requires tedious and difficult alignment procedures.
Past optical systems formed the output of a laser diode into a single mode, near diffraction limited beam using a collection of components (lenses, microlenses, diffraction gratings, prisms, coatings, binary optical elements, GRIN optical elements, and combinations of refractive and diffractive (binary) optics) assembled together by mechanical means and aligned by various optical and interferometric means. Many such embodiments of collimators and other beam shaping optics for diode lasers have been described in the literature and in various patents. Diodes with several of these previous embodiments are available from various manufacturers. Most of these embodiments are quite costly because of the manpower intensive assembly and alignment requirements. Most such embodiments also fall short of desired performance.
The present embodiment of an integrated molded plastic optical system comprises a four surface refractive/diffractive combination to effect the collimation of a laser diode into a near diffraction limited circularized beam. Laser diodes are typified by a fan-like output characteristic of their slit-like radiating aperture.
The specific design of this embodiment effects a narrow, collimated and circularized beam for potential application to laser bar code readers or similar devices. This embodiment also, coincidentally is comprised of a combination of anamorphic refractive surfaces and a diffractive surface. The design collimates, corrects for astigmatism and is achromatic over a significant waveband. The achromatic feature allows the same device to be used with diode lasers of a variety of wavelengths and still deliver near-diffraction limited performance. Near diffraction limited performance requires tolerances on alignment and on all optical surfaces to be such that the output wavefront deviates from a diffraction limited beam by less than {fraction (1/40)}th of a wavelength.
A specific embodiment depends on the desired application. In some cases a small collimated and circularized beam such as the present embodiment may be desired. In others a larger diameter, more slowly diverging collimations is preferred. Sometimes a focused spot is desired, in others re-imaging of the slit aperture with some magnification is desired. Each such embodiment requires a different optical design. Moreover, a given application can have an infinite number of optical design solutions.
Use of molded optics, both of glass and plastic, have been proposed. However, at the time of these proposals, residual stresses in the material resulting from the molding processes resulted in refractive properties that were inadequate to the task of delivering aberration free optical beams. Recent developments in plastic optics have effected components with sufficient precision that discrete component devices can now be built of molded optics. These recent developments are essential to the processes involved in all embodiments of integral molded plastic optical systems.
There is provided by this invention a variety of processes that culminate in molded plastic pieces that are directly assembled and bonded without external mechanical or alignment aids to comprise complete integrated molded plastic optical systems. These processes include the following:
1. Optical design processes to optimize a given design for the precision molding process and subsequent simple assembly.
2. Processes to manufacture the optical masters, including ashperes, anamorphic surfaces and diffractive surfaces into hard metals with the necessary precision to effect the precision mold.
3. Processes to effect the precision mold that includes sliding precision pins in the mold cavities that form the plastic components with the built in alignment aids.
4. Processes that include optical pins with the negatives of the desired optical surfaces for molding aspheric and/or anamorphic refractive components.
5. Processes that include optical pins with the negative of diffractive surfaces to effect the required molding of these surfaces.
The essence of the invention is that precision manufacture, assembly and alignment is required only once and is applied to the optical masters, the mold cavity, the sliding pins and their subsequent assembly and alignment. Once this is performed any number of optical systems can be produced, assembled and bonded into an integrated optical system without mechanical or alignment aids. Thus the manufacturing costs associated with precision manufacturing, assembly and alignment is spent only once and is amortized over a large number of systems. The result is higher quality systems at lower cost.
The present embodiment of the invention is a mold for mass production of a Laser Diode Collimating and Aberration Correction (LDCC) plastic optical system.
One object of the invention is to place an optical system that is composed of several optical surfaces (aspheric refractive surfaces and a non-axisymmetric diffractive surface) aligned relative to each other in a configuration that is placed in front of a laser diode and positioned as a single optical element such that the optical system is aligned relative to the emitter of the diode. The optical system will collimate the output from the emitter of the laser diode such that the laser beam is collimated to diffraction limited performance and that the output beam is circularized and corrected for the astigmatism.
Another object of the present invention is that the optical system has been achromatized in order to compensate for laser wavelength drift.
Another object of the present invention is that the optical elements shall be optimized with respect to size in order to create a compact system that has a reasonably smaller output beam than is possible with conventional lens systems. All of this must be accomplished outside of the laser diode package and be reasonable to align to the diode without complex special kinematic optical positioning equipment.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate a preferred embodiment of the invention.